The so-called air blown fiber system (ABF) is characterized by a fiber laying process that comprises blowing a fiber bundle through a tube cable using compressed air or dry nitrogen. Lightweight and flexibility are required of such fiber bundle used in an ABF system. The main advantage of the air blown fiber system is that a long fiber bundle can be blown through a tube cable laid in a given network and, if necessary, the fiber bundle can be discharged from the tube cable and reused. Due to such advantage, the ABF system has been applied to optical fiber wiring for indoors, in particular, network systems in high-rise buildings.
A fiber unit used in the ABF system comprises a fiber or ribbon bundle, and a sheath layer coated thereon, the sheath layer being composed of two layers, i.e., an inner tight bundle coating layer which tightly wraps the fiber or ribbon bundle and a lightweight outer layer which imparts high strength and good flexibility to the fiber bundle. In the past, the two layers of the fiber unit have been formed by injection molding or by a combination of a UV-curing process for the inner layer and an injection/foaming process for the outer layer. Since, the productivity of the injection method is lower than that of the UV-curing process, there has recently been suggested a method of applying the UV curing process to the outer coating layer. For example, U.S. Pat. Nos. 5,557,703, 5,563,985, 5,903,693, 5,970,195, 6,101,304 and 6,249,629 disclose a method of forming a low-density outer layer by admixing a lightweight microsphere composition with a UV-curable resin. However, this method has disadvantages in that such a microsphere composition has to be mixed right before curing and the resulting resin coating is not uniform.
Thus, there has been a need to develop a UV-curable resin composition which can be used to form a light-weight, homogeneously foamed secured layer.